1. Field of the Invention
This invention relates generally to an apparatus and method for temporarily plugging a tubular in a well penetrating a subterranean formation, and in particular to an apparatus and method for controlling the pressure at which a fluid is introduced into the well and/or the formation whereby the plug opens upon application of a predetermined pressure.
2. Description of Related Art
When conducting an operation within a well penetrating a subterranean formation, it is often desirable to initiate the operation after particular conditions are met within the well. For example, it is desirable in hydraulic fracturing to introduce a fluid at a predetermined pressure simultaneously to a series of existing perforations in a well. Another example is the placement of packers in the desired position prior to establishing a seal between the packer and the well casing and tubing or other hardware in the well thus, a temporary plug may be installed within or at the end of a tubular and then opened when the desired conditions are met. Blow out subs attached to tubulars are commonly used to carry out such operations. A blow out sub is a device having a valve or plug which can be opened to provide fluid communication between a space within the tubular and the exterior of the tubular.
In many blow out subs, hydraulic pressure is utilized to rupture or shear a plug which prevents fluid flow. One type of plug is a rupture disk composed of a frangible material which shatters or fails when the pressure difference between the two sides of the disk exceeds a particular value. Large diameter, high strength rupture disks are generally expensive and difficult to manufacture. Therefore, rupture disks tend to be utilized in applications where the disk diameter and/or pressure differential are relatively small. Another type of plug comprises a shear pin connecting a stationary member, such as a wall or housing, and a moveable member, such as a piston. The shear pin fails when the pressure differential across the piston reaches a certain value, and the piston then moves so as to equalize the pressure on either side. Shear pins can be manufactured in a wide range of strengths, and the applied force can be distributed between 2 or more pins.
Blow out subs commonly include plugs designed to fail on the basis of the pressure difference between the fluid in the tubular above the plug and the fluid below the plug. In oil field operations, the pressure above the plug is generally well known, but the pressure below the plug is often poorly known or estimated. For example, bottom hole pressure measurements may not have been performed in a new well, or the time required for pressure buildup after a well is shut in may not be known. Thus, the well pressure may be unknown at the time an operation is conducted. If the estimated pressure below the plug is incorrect, the plug may open at the wrong time, or it may not shear at all, possibly causing severe adverse consequences for the attempted operation. For example, a particular fluid pressure is required for successful hydraulic fracturing of a subterranean formation. If the fluid pressure below the plug is underestimated, the plug may open when the pressure above the plug is too low to fracture the formation. Thus, a need exists for installing a plug which opens reliably when the fluid pressure in the tubular reaches a predetermined value, independent of the pressure outside the tubular. The plug may be mounted in a blow out sub.
Devices intended to shear or fail upon application of pressure in excess of a predetermined value are also used to detect overpressure situations in industrial process equipment. For example, a chamber between two frangible disks is filled with a fluid at a known pressure, such as atmospheric pressure. The disks rupture when the pressure in the industrial process equipment exceeds a predetermined absolute value. However, these pressure monitors serve different functions than blow out subs. The pressure monitors are intended for use in side branches of pressurized fluid systems, while blow out subs are utilized in the direct fluid flow path. In addition, the industrial process monitors operate in environments with significantly lower pressures than would be expected in petroleum wells, where blow out subs are utilized.
Thus, it is an object of the present invention to provide a means by which a plug in a blow out sub may be opened upon attainment of a precise, predetermined absolute pressure in the tubular above the plug.
It is yet another object of the present invention to provide a means by which a plug in a blow out sub may be opened to introduce a second fluid into a well upon attainment of a precise, predetermined absolute pressure in the tubular above the plug after introduction of a first fluid.
It is a further object of the present invention to provide a method for controlling the pressure at which a fluid is introduced into a well penetrating a subterranean formation.